Rotaxanes template-directed synthesis

If during the template-directed synthesis of a rotaxane, the location of two identical recognition sites ( stations ) within its dumbbell component can be arranged (Fig. 13.3a and b), a degenerate, coconformational equilibrium state is obtained in which the macrocyclic component spontaneously shuttles back and forth between... 

Figure 19. The template-directed synthesis of the [2]rotaxane 56-4PF6.

Raymo, F.M., Stoddart, J.F. Organic Template Directed Synthesis of Catenanes, Rotaxanes and Knots. In ref. 4b, p.143. 

The -electron-deficient pyridinium ring has played an important role in template-directed synthesis in which the interaction of a -electron acceptor and a -electron donor facilitate self-assembly. This has led to the construction of a variety of supramolecular assemblies including catenanes (linked rings) and rotaxanes (wheel and axle) . 

A template-directed synthesis of [2]-rotaxanes, with a yield in one case of 72%, has been reported. The experiments employed dumb-bell-shaped components incorporating terminal triisopropylsilyl stoppers connected to a central 1,5-dioxynaph-thalene recognition site by [-CH2CH20-] spacers (n = 1-3). - These components were used as templates for the synthesis ( clipping reaction ) of the corresponding rotaxanes incorporating cyclobis(paraquat-p-phenylene) as the ring component. The... 

Stoddart and co-workers transferred the strategy of template-directed synthesis to the formation of the first molecular system consisting of five interlocked rings in a linear array (Structure 51). They suggested that this [5]catenane should be called olympiadane [64]. Recently they reported the self-assembly of a new type of rotaxane, in which three side chains are linked directly to a single central core, producing a dendritic-type structure [65]. 

A similar recognition motif has been employed by Vogtle et al. [8,20] and by Leigh et al. [21, 22] in the template-directed synthesis (Figure 3-5) of [2]rotaxanes. The glycine-based compounds 15 and 16, incorporating terminal diphenylmethane stoppers, are able... 

Figure 3-8 Template-directed synthesis of cyclodextrin-containing [2]rotaxanes 33 and 34.

This strategy was extended [46] (Figure 3-20) to the template-directed synthesis of a [2]rotaxane incorporating a dumbbell-shaped component possessing one ammonium and... 

Figure 24 Syntheses of receptor 53 and rotaxane 91 using template-directed synthesis.

The axle and ring components usually exhibit some kind of interaction originating from complementary chemical properties, which is also exploited in the template-directed synthesis of such systems. In rotaxanes containing two different recognition sites in the axle component, it is possible to switch the position of the ring between these two stations by an external stimulus. Systems of this type, termed molecular shuttles, " constitute the most common implementation of the molecular motor concept with rotaxanes. 

Ashton, P.R., Johnson, M.R., Stoddart, J.F., Tolley, M.S., and Wheeler, J.W. (1992) The template-directed synthesis of porj yiin-stoppered [2]rotaxanes, J. Chem. Soc., Chem. Commun. 1128-1131. 

Griffiths, K. E. and Stoddart, J. F. Template-directed synthesis of donor/acceptor [2]catenanes and [2] rotaxanes. PureAppl. Chem. 80, 485-506, 2008. 

Fortunately, more efficient methods for the complexation of macrocyclic hosts with acyclic guest molecules have become available with the advent of supramo-lecular chemistry, resulting in higher yields in rotaxane and catenane synthesis. In the following sections, the preparation of different types of interlocked molecules, with the use of host-guest recognition, is discussed. It should be noted that these template-directed methods differ significantly from the above-mentioned stochastic approach [28]. 

Tripincer template-directed cyclization of a dienic pyridine precursor leads to the formation of the 69-membered ring trimeric heterocycle 112 (Figure 2.6). Decomplexation of the tripincer from the product is realized with NaCl [72]. The synthesis of a [2]rotaxane by M-RCM using a similar pincer-pyridine complex for pre-assembly was also reported by Leigh et al. [73]. 

---